The present invention relates to a dose detector having a function of checking a semiconductor radiation detector for measuring a radiation; dose in a radioactive isotope handling facility such as a nuclear facility and a laboratory, and in a natural environment. The present invention also relates to a dosimeter provided with the dose detector.
A dosimeter measures and, displays radiation doses (an exposure dose or a dose) in real time to find that a nuclear personnel is exposed during work, and generates an alarm when the exposure dose exceeds a predetermined limit, thereby preventing the nuclear personnel form being overexposed to radiation. The nuclear personnel always carries the dosimeter while working in a radiation controlled area, and often carries the dosimeter in a pocket of a work clothe.
In an environment in which the dosimeter is used, there are low level radiations other than the radiation to be measured. The low level radiations include naturally existing radiations, low level radiations from a radioactive material on the earth, in-vivo radioactivity, and cosmic rays. The nuclear radiations other than the nuclear radiations to be measured are referred to as background. The dosimeter detects the unnecessary background, and the background is generally removed as a noise before detecting the nuclear radiations to be measured. In the specification, an output of the semiconductor radiation detector based on the background will be referred to as background noise.
A conventional dosimeter has a noise removal function for removing the background noise. The noise removal function for removing the background noise will be explained with reference to FIG. 15. FIG. 15 is a block circuit diagram showing a conventional dose detector mounted in the conventional dosimeter. The conventional dose detector shown in FIG. 15 has the noise removal function.
As shown in FIG. 15, a dose detector 100 includes a semiconductor radiation detector 101, an amplifier 102, a pulse height discriminator 103, and a counter 104. The dose detector 100 also includes a central processing unit (CPU; not shown) and a unit for processing data (not shown). The semiconductor radiation detector 101 outputs a detection signal according to detected nuclear radiations. The amplifier 102 amplifies the detection signal with a predetermined gain to adjust an amplitude voltage (wave height) of the detection signal appropriately.
The pulse height discriminator 103 is a discriminator circuit for inputting a reference detection wave height voltage (a reference detection voltage) determined by voltage dividing resistors 103a and 103b to a comparator circuit 103c, and discriminates a signal with a wave height voltage higher than the reference detection voltage from the detection signal, so that the discriminated signal is output as a pulse signal. Through the discrimination according to the wave height, it is possible to remove the background noise below the reference detection voltage. The counter 104 counts the pulse signal after the background noise is removed, thereby counting the radiation dose.
In the dose detector 100, when the pulse height discriminator 103 is not provided, the counter 104 may output a large number of count values including the background noise below the reference detection voltage, as shown as a count value to wave height voltage characteristic in FIG. 16. When the pulse height discriminator 103 discriminates the detection signal according to the wave height, the signal with a voltage above the reference detection voltage is output as the pulse signal, and the background noise below the reference detection voltage is removed as an object not to be detected, thereby detecting only the dose of the nuclear radiations to be detected.
Such a conventional device has been disclosed in Patent Document 1. Patent Document 2 has disclosed a structure in which a semiconductor radiation detector has a self-check function.
Patent Document 1; Japanese Patent Publication (Kokai) No. 09-33660
Patent Document 2; Japanese Patent Publication (Kokai) No. 59-46573
In the conventional dosimeter having the dose detector 100, it is checked whether the dose-detector 100 is working properly at specific intervals. For example, in the dosimeter shown in FIG. 15 or a beta- or β-ray dose measuring instrument disclosed in Patent Document 1, the dose detector is exposed to a detector calibration radiation source (a checking source) 200 with a specified distance in between. After the dose detector is exposed to a predetermined irradiation dose of radiation from the checking source 200 for a predetermined period of time, when a count value is a predetermined value, the dose detector is judged to be normal. When the count value is smaller than the predetermined value (0 or close to 0) or too large, the dose detector is judged to be abnormal.
It is necessary to carefully handle the checking source 200 due to radioactive isotope, thereby making is difficult to check the dosimeter safely. Also, it is troublesome to handle the checking source 200 due to history management, a strict facility for storing the radioactive isotope and access record management. It is necessary to interrupt use of the dosimeter for checking whether the dosimeter is functioning, thereby taking long time.
The semiconductor radiation detector disclosed in Patent Document 2 is capable of self-checking. A light emitting device irradiates the semiconductor radiation detector to check an operation thereof, thereby making it possible to check the detector without the checking source 200. However, it is necessary to provide the light emitting device, a driver circuit for driving the light emitting device, and a judging unit in the semiconductor radiation detector, thereby limiting a design of the semiconductor radiation detector. It has been desired to obtain a checking function with a simple structure based on a different concept.
In view of the problems described above, an object of the present invention is to provide a dose detector in which it is possible to safely check the dose detector with a simple structure.
Another object of the present invention is to provide a dosimeter provided with the dose detector.
Further objects and advantages of the invention will be apparent from the following description of the invention.